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Electrochimica Acta 306 (2019) 360e365

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Electrochimica Acta

journal homepage: www.elsevier .com/locate/e lectacta

Interface-induced controllable synthesis of Cu2O nanocubes forelectroreduction CO2 to C2H4

Wenhang Wang a, Hui Ning a, **, Zhongxue Yang a, Zhaoxuan Feng a, Jialin Wang a,Xiaoshan Wang a, Qinhu Mao a, Wenting Wu a, Qingshan Zhao a, Han Hu a, Yan Song b,Mingbo Wu a, *

a State Key Laboratory of Heavy Oil Processing, Institute of New Energy, College of Chemical Engineering, China University of Petroleum (East China),Qingdao, 266580, Chinab CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China

a r t i c l e i n f o

Article history:Received 29 November 2018Received in revised form12 March 2019Accepted 20 March 2019Available online 23 March 2019

Keywords:CO2 electroreductionGraphite sheetsIonic liquidEthyleneCuprous oxide

* Corresponding author.** Corresponding author.

E-mail addresses: ninghui@upc.edu.cn (H. Ning), w

https://doi.org/10.1016/j.electacta.2019.03.1460013-4686/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Electroreduction CO2 to high value-added chemicals is a promising way to mitigate the greenhouse effectwhile storing the renewable electricity in chemicals. As an important chemical, C2H4 is a desirableproduct of CO2 reduction but limited by lacking of efficient and durable catalysts. Herein, we proposed aninterface-induced method to prepare Cu2O nanocubes with controllable morphology and size. Due to theimidazolyl groups connected to the surface of ionic liquid functionalized graphite sheets (ILGS), Cu2Onanocubes were induced to grow on ILGS to obtain Cu2O/ILGS composites, where the size of Cu2O can becontrolled by adjusting the concentration of Cu2þ. Interestingly, as the concentration of Cu2þ increasesfrom 12.5mmol/L to 100mmol/L, the size of Cu2O nanocubes decreases from 456 nm to 72 nm. Ascatalyst for CO2 electroreduction in 0.1M KHCO3 aqueous solution, Cu2O/ILGS-100 (synthesized under100mmol/L CuCl2) behaves the best catalysis performance with a high faradic efficiency of C2H4 (31.1%)and long durability at �1.15 V (vs. reversible hydrogen electrode).

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Electroreduction of carbon dioxide (CO2RR) to high value-addedchemicals offers an opportunity for alleviating the greenhouse ef-fect and storing the renewable electricity in chemicals [1e4].Recent progresses in electroreduction of CO2 mainly focus on theproduction of gaseous carbon products, such as carbon monoxide(CO) [5,6] methane (CH4) [7,8] and ethylene (C2H4) [9,10], of whichC2H4 is of significant value but limited by lacking of efficient anddurable catalysts [11].

Among all the screened materials, copper-based materials haveshown the best performance for electrocatalytic CO2 to hydrocar-bons, especially for C2H4 [12e15]. Recently, Cu2O is found to havehigh selectivity to C2H4 in electrocatalytic CO2RR, whose perfor-mance is closely related to its morphology and size [16e20]. Recentresearch has proved that the surface of Cu2O is reduced to Cu0 with

umb@upc.edu.cn (M. Wu).

O vacancies during the CO2RR [21], which is greatly affected by theapplied potentials and currents [22]. The high C2H4 selectivitybrought by Cu2O catalysts is more likely due to a high density ofgrain boundaries with plenty of unsaturated coordinated copperatom with suitable atomic spacing. Liu et al. [23] proposed a metalembedded in oxidized matrix model with quantum mechanicsmethods to explain the activity and selectivity of Cu2O in electro-catalytic CO2RR, indicating that the Cu2O matrix plays the key rolein maintaining the high performance of oxide derived Cu0.

As a result, construction of Cu2O crystals with suitablemorphology and size to exposure plenty of unsaturated coordi-nating Cu atoms are crucial to improving the activity and selectivityof Cu2O for electrocatalytic CO2RR to C2H4. Unfortunately, thesynthesis of well-defined and highly dispersed Cu2O nanocrystals isstill a great challenge. Huang et al. prepared Cu2O nanocubes bycontrolling the aggregation of Cu2O seed particles and surfacereconstruction method via sodium dodecyl sulfate as capping sur-factant [24]. Yeo et al. reported copper(I) oxide films electro-deposited from an electrolyte of 0.3mol/L CuSO4, 3.2mol/L NaOHand 2.3mol/L Lactic acid in water at various electrochemical po-tentials [25]. Wang et al. prepared Cu2O hollow spheres by

W. Wang et al. / Electrochimica Acta 306 (2019) 360e365 361

adjusting the concentration of CTAB in water under 60 �C [26].These reported methods usually need trivial steps and massiveenergy consumption, bulk organic solvents or mass surfactants,which are environmentally unfriendly. Therefore, it is of greatsignificance to invent an economic method for controllable syn-thesis of Cu2O nanocrystals with high catalytic performance forC2H4 production from CO2RR.

The interface-induced method is an outstanding strategy forcontrollable preparation of metal materials [27]. Carbon materialsgenerally possess excellent chemical and thermal stabilities, goodelectrical conductivity [28], and flexible designability, making thempromising candidates as supports to synthesize metal based com-posites based on interface-induced method [29,30]. Pan et al.developed a facile interface-induced Zn(II)-ligand-fragment co-assembly strategy to in situ fabricate boronate-based MOF mem-brane on hydrophobic porous carbon substrate for specific molec-ular recognition and separation. A catechol-containing medicinalnatural flavone luteolin was found to be efficiently and selectivelyrecognized on theMOF composite inwater-containing solution dueto the phenylboronic acid groups and carbon substrate [31]. Renet al. employed the graphene oxide (GO) support to template Cu2Onanocrystals resulting in well-dispersed Cu2O nanocrystals thatshow much promise in photo and chemo-catalytic applications,where GO play roles as the template and a surfactant to achievedispersed Cu2O crystals [32]. These preliminary works motivate usto investigate the interface-induced method with carbon materialsfor controllable synthesis of Cu2O nanocubes as high efficientcatalyst for ethylene production from CO2RR.

Herein, the ionic liquid functionalized graphite sheets (ILGS)were prepared by electro-exfoliation method according to ourprevious works [33], which was dispersed into aqueous solution toconstruct ILGS-water interface. Under the interface effects of ILGSon Cu2þ, a series of well-defined Cu2O nanocubes were in situfabricated on the surface of ILGS, in which the morphology and sizeof Cu2O were completely different with those without ILGS. The as-prepared Cu2O/ILGS showed significant enhancement of catalyticperformance for ethylene formation in CO2RR.

2. Experiment

2.1. Materials

1-octyl-3-methylimidazolium chloride (OmimCl, 99.0%) wassupplied from the Centre of Green Chemistry and Catalysis, LICP,CAS, P. R. China. Copper chloride dihydrate (99.0%), sodium hy-droxide (96.0%), potassium hydrogen carbonate (99.5%), ethanolanhydrous (99.7%) and L-ascorbic acid (99.99%) are provided bySinopharm Chemical Reagent Co., Ltd., P. R. China. Nafion N-117membranes (0.180mm thick, �0.90 meq∙g�1 exchange capacity)were purchased from Alfa Aesar China Co., Ltd. High purity graphiterod (ø¼ 3mm) was provided by Beijing Crystal Dragon CarbonTechnology Co., Ltd., P. R. China. All the reagents are used asreceived without further treatments. The water was purified by aMillipore system in all experiments.

2.2. Synthesis of Cu2O/ILGS

Cu2O/ILGSs were prepared using a simple wet chemicalapproach. Firstly, ILGS was synthesized according to our previouswork [33]. Then, 40mg ILGS is added into 40mLwater to obtained awell-dispersed suspension under ultrasonic for 30min at roomtemperature, denoted as solution A. After that, 12mL of 12.5mmol/L CuCl2 aqueous solution was mixed with solution A and stirredvigorously for 1 h, flowing by adding 20mL of 0.1mol/L NaOHslowly under stirring to obtain solution B. Subsequently, 50mL of

6mmol/L L-ascorbic acid is added to solution B and stirred for 2 h atroom temperature. At last, the precipitate was filtered and washedwith water and ethanol thoroughly before drying under vacuum at60 �C for 6 h to obtain a brown power, denoted as Cu2O/ILGS-12.5.Cu2O/ILGS-25, Cu2O/ILGS-50, and Cu2O/ILGS-100 were preparedwith the same procedure except that the concentration of CuCl2 is25, 50 and 100mmol/L, respectively. The yields of synthesismethod, the nominal and real amount of Cu2O obtained with themethod were listed in Table S1.

As control experiments, pristine Cu2O-12.5, Cu2O-25, Cu2O-50and Cu2O-100 were also prepared with the same procedurewithout adding ILGS in solution A, where the number after theCu2O represented the concentration of Cu2þ (mmol/L).

2.3. Materials characterization

The morphologies of materials were characterized by scanningelectron microscopy (SEM, Hitachi S-4800, Japan). The crystalproperty and phase composition of all as-prepared materials wereinvestigated by X-ray diffraction (X'Pert PROMPD, Holland) with CuKa radiation at 40 kV and 40mA with Cu Ka radiation(l¼ 1.5406Å). The surface elemental compositions were recordedby X-ray photoelectron spectroscopy (XPS, Thermo ScientificEscalab 250XI, America) with Al Ka radiation. UVeVis absorptionspectra were measured by a UVeVis spectrophotometer (UV-2700,SHIMADZU). Thermogravimetric analysis (TGA) was carried on aShimadzu TA-60Ws thermal analyser (Japan) in air at a ramp rate of10 �C min�1.

2.4. CO2RR and product analysis

The CO2RR was carried out in an H-type electrolytic cell withtwo compartments, where the anode and cathode were separatedby a Nafion 117 proton exchange membrane. A Pt plate (1� 1 cm2)and Ag/AgCl electrode (KCl saturated) were used as counter andreference electrode, respectively. Both compartments were filledwith 30mL of 0.1MKHCO3 as electrolyte. All theworking potentialswere referred to the reversible hydrogen electrode (RHE) [34]. ThepH value of the CO2 saturated electrolyte is 6.8. All the electro-chemical measurements were performed using an electrochemicalworkstation (CHI 760, Shanghai CH Instruments Co., P. R. China).

The working electrode was fabricated by drop-casting 100 mL ofcatalyst ink onto a L-type glassy-carbon (GC, ø¼10mm) and driedwith N2. Before ink deposition, the GC electrode was polished andsonicated in ethanol. The catalyst inks were prepared by dispersing5.0mg materials in 10 mL of 5% Nafion solution and 500 mL ofethanol anhydrous.

Before testing, the electrolyte in cathode side was saturatedwith N2 for 30min to exclude air. Then high purity CO2 (99.999%)was bubbled into the electrolyte for another 30min. During theelectrochemical measurements, the CO2 gas flow was controlled at20mL/min. The gas-phase products generated during CO2 elec-trolysis at each fixed potential were collected and quantitativelyanalyzed by gas chromatograph (BFRL-3420A, China), which wasonline connected with the head space of the cathodic side of the H-type cell. The gas products were injected to GC using a ten-portvalve system with high purity Ar (99.999%) as carrier gas. The GCsystem was equipped with two columns associating with two de-tectors. The thermal conductivity detector (TCD) was installed todetect hydrogen and carbon monoxide while the flame ionizationdetector (FID) was fabricated to detect hydrocarbons. The liquidproduct was tested with Nash's colorimetric method by UV-Visspectrophotometer [35]. The faradaic efficiency of all products wascalculated according to the methods reported by Yeo et al. [25]

Fig. 1. XRD patterns of Cu2O/ILGSs.

W. Wang et al. / Electrochimica Acta 306 (2019) 360e365362

3. Results and discussion

The typical preparation procedure is schematically illustrated inScheme 1. The XRD patterns (Fig. 1) and XPS spectra (Fig. 2) indicatethat Cu2O nanocrystals are successfully synthesized and in-situsupported on the ILGS. As shown in Fig. 1, the wider diffractionpeak at 26.43� is ascribed to the (002) peak of ILGS due to theincreased amorphousness and defects after exfoliation of graphite.Other diffraction peaks can be exactly indexed to Cu2O (JCPDSNo.78-2076) at 29.58� (110), 36.44� (111), 42.33� (200), 52.49�

(211), 61.40� (220), and 73.56� (311), respectively. No other char-acteristic peaks exit in the XRD patterns, indicating Cu2O wassuccessfully synthesized and in-situ supported on ILGS.

It can be seen from the XPS spectra (Fig. 2a) that all Cu2O/ILGSsare consist of C, N, O and Cu. In Fig. 2b, the peaks at 399.57 eV and401.06 eV indicate that nitrogen in ILGS is derived from the cationsof 1-octyl-3-methylimidazolium chloride, which is used as elec-trolyte for graphite electro-exfoliation [36]. In Fig. 2c, the charac-teristic peaks at 932.1 eV and 952.0 eV are corresponding to Cu2p3/2and Cu2p1/2, respectively. The specific valence of Cuþ and Cu0 wasfurther investigated by Cu LMM Auger spectra (Fig. 2d), the kineticenergy around 916.9 eV confirms the purity of Cu2O without Cu0

exiting [37].SEM images revealed that all the Cu2O nanocrystals present

cubic morphology with obviously different size distributions, asshown in Fig. 3 and Fig. S1. To be specific, obvious cavities on thesurface of Cu2O nanocubes was found in Cu2O/ILGS-12.5 due toincomplete growing, which is beneficial for the exposure of un-saturated coordinate Cu atoms. The size distribution of Cu2Onanocubes supported on ILGS showed an abnormal rule in com-parison with pristine Cu2O. As shown in Fig.S2 and Table S2, withthe concentration of Cu2þ increases from 12.5mmol/L to100mmol/L, the average side length of Cu2O in Cu2O/ILGSs de-creases from 456 nm to 72 nm, while the average side length ofpristine Cu2O increases from 113 nm to 228 nm, indicating the ILGScan inhibit the growth and aggregation of Cu2O crystals under highconcentration of Cu2þ.

The catalytic performances of Cu2O/ILGSs for CO2 electro-reduction to C2H4 were investigated with CO2-saturated 0.1MKHCO3 solution as electrolyte in an H-type cell under series po-tentials. The faradic efficiency of C2H4 and other gas products weredepicted in Fig. 4. For all Cu2O/ILGSs, the optimum values of fara-daic efficiency for C2H4 are at �1.15 V (vs. RHE). Cu2O/ILGS-100shows a maximum faradaic efficiency (FE) of 31.1% for C2H4 prod-ucts. As shown in Fig. S3, formate as the only liquid product wasdetected with a combined FE of ca. 100% with gas products forCu2O/ILGS-100.

Scheme. 1. Illustration for pr

In our previous work, ILGS has been proved to have no catalyticactivity for electroreduction of CO2 while Cu2O is the main activesites. For pristine Cu2O (Fig. 4e), along with the average size of Cu2Oincreasing from 113 nm to 228 nm, the FEC2H4 declines from 17.5%to 13.8%. Similar trends can also be found for Cu2O/ILGS-25, Cu2O/ILGS-50 and Cu2O/ILGS-100. Reasonably, the highest FEC2H4 (31.1%)was obtained over Cu2O/ILGS-100 due to the smallest size of Cu2Onanocubes (72 nm). Specially, the FE of C2H4 on Cu2O/ILGS-12.5 is25.5%, which is close to Cu2O/ILGS-25 (25.0%), though the size ofCu2O in Cu2O/ILGS-12.5 is much larger than that in Cu2O/ILGS-25.This special phenomenon is corresponding to the specialmorphology of Cu2O with obviously cavities on the crystal surface(Fig. 3a), whichmay arise from the interface-induced effects of ILGSassociating with the low concentration of Cu2þ. These concavitiesincrease the density of grain boundaries of Cu2O nanocubes withmore unsaturated copper atoms exposed, which is conducive toenhancing the selectivity of C2H4 [38]. Among all the materials asmade, the Cu2O nanocubes in Cu2O/ILGS-100 have the largestloading of 77.6wt% (Fig. S4, Table S3) and the smallest size of 72 nm,indicating this is a facile method for rapid synthesis of smaller Cu2Onanocubes using high concentrations of Cu2þ for industrialapplications.

It is worth noting the mass activity of Cu2O towards ethylene

eparation of Cu2O/ILGS.

Fig. 2. (a) XPS spectra of Cu2O/ILGSs, (b) High-resolution N1s spectra of Cu2O/ILGS-12.5, (c) High-resolution Cu2p spectra of Cu2O/ILGSs, (d) Cu LMM Auger spectra of Cu2O/ILGSs.

Fig. 3. SEM images of (a) Cu2O/ILGS-12.5, (b) Cu2O/ILGS-25, (c) Cu2O/ILGS-50, (d) Cu2O/ILGS-100.

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formation is meaningful for evaluating the real catalytic activity ofcopper. Under the optimum potential �1.15 V (vs. RHE), the for-mation rate of ethylene on unit mass of Cu2O was calculated, asshown in Fig. S5. Not surprisingly, Cu2O/ILGS-12.5 presents the bestperformance due to its lowest loading of Cu2O with obvious

concavities, flowing by Cu2O/ILGS-25 and Cu2O/ILGS-50. Interest-ingly, Cu2O/ILGS-100 exits a relative high yield of C2H4, much betterthan Cu2O/ILGS-50 and Cu2O/ILGS-25, with the highest Cu2Oloading. With further to explain this phenomenon, the BET surfacearea of different materials as-made was calculated according to N2

Fig. 4. Faradaic efficiencies of gas products from CO2RR on (a) Cu2O/ILGS-12.5, (b) Cu2O/ILGS-25, (c) Cu2O/ILGS-50, (d) Cu2O/ILGS-100. (e) Comparison of FEC2H4 on Cu2O/ILGSs andCu2Os at �1.15 V (vs. RHE). The insets illustrate the morphology and size of Cu2O. (f) Stability test of Cu2O/ILGS-100 and Cu2O-100 at �1.15 V (vs. RHE).

W. Wang et al. / Electrochimica Acta 306 (2019) 360e365364

adsorption-desorption isotherms. As depicted in Fig. S6, the BETsurface area of Cu2O/ILGS-12.5, Cu2O/ILGS-25, Cu2O/ILGS-50 andCu2O/ILGS-100 is 5.90, 5.23, 6.07, and 6.30m2 g�1, respectively,which is closely related to the size and morphology of Cu2Onanoparticles. Cu2O/ILGS-100 has the largest specific surface areadue to its smallest size of Cu2O crystals. However, Cu2O/ILGS-12.5has a larger specific surface area than Cu2O/ILGS-25, even thoughthe latter has a much smaller size of Cu2O. This is consistent withthe special morphology of Cu2O crystals in Cu2O/ILGS-12.5, whichhave obvious surface cavities (Fig. 3a). As revealed by the linearsweep voltammetry (LSV) curve in Fig. S7a, Cu2O/ILGS-100 ach-ieved the highest total current density from �0.85 V to �1.3 V (vs.RHE). According to the mass content of Cu supported on ILGS, themass-specific current density of all the catalysts was tested andnormalized by 1.0mg copper, as shown in Fig. S7b. It is found thatCu2O/ILGS-100 still has the largest current density under themeasured potentials, indicating that ILGS as supports facilitate theutilization of Cu atom as catalyst for CO2 electroreduction.

In the end, the stabilities of Cu2O/ILGS-100 and Cu2O/ILGS-12.5were selected as representative samples to investigated the dura-bility of Cu2O under �1.15 V (vs. RHE), as depicted in Fig. 4f andFig. S8, respectively. Despite the great difference of Cu2O nanocubesin size and morphology, both Cu2O/ILGS-100 and Cu2O/ILGS-12.5

showed better durability than the corresponding pristine Cu2O,indicating the interface-inducedmethod is preferable to synthesizeCu2O materials with higher selectivity and better durability forelectrocatalytic reduction of CO2 towards C2H4. We furtherexplored the morphology and state of Cu2O after CO2 electro-reduction. Taking Cu2O/ILGS-100 for example, the edge of Cu2Onanocubes is blurred (Fig. S9), indicating themorphology of Cu2O isnot stable during the CO2 electroreduction [39]. As shown inFig. S10, the Cu 2p XPS showed that Cuþ or Cu0 exists in Cu2O/ILGS-100 and no Cu2þ peaks were observed. With further analyzed formthe Auger Cu LMM line (Fig. S11), it is concluded that only minimalCuþ was reduced to Cu0 under the applied negative potential andCuþ is still themain active species in Cu2O/ILGS-100, which play thekey role in selectively catalytic CO2RR to ethylene [19].

4. Conclusions

In summary, an interface-induced method was developed tosynthesize Cu2O nanocubes with controllable morphology and size.Due to the nanostructured surface functionalities of ILGS, Cu2Onanocubes with smaller size and high loading amount on ILGSwereprepared. As the concentration of Cu2þ decreases from 100mmol/Lto12.5mmol/L, the size of Cu2O on ILGS increases from 72 nm to

W. Wang et al. / Electrochimica Acta 306 (2019) 360e365 365

456 nm, while the size of pristine Cu2O without ILGS as supportsdecreases from 228 nm to 113 nm. This abnormal phenomenonwasascribed to the interface effects of ILGS: (1) The Cu2O nucleus ispreferable to generate on the surface of ILGS rather than in the bulksolution due to the absorption effect of ILGS to Cu2þ; (2) Thenanostructures of ILGS can fix the position of Cu2O crystals andinhibit their aggregation. Under the optimal potential for C2H4formation (�1.15 V vs. RHE), all Cu2O/ILGSs show higher selectivitytowards C2H4 than the corresponding pristine Cu2O prepared withthe same concentration of Cu2þ, of which Cu2O/ILGS-100 exhibitsthe highest faradic efficiency of ethylene (31.1%). The high perfor-mance of Cu2O/ILGS-100 is attributed to its larger electrochemicallyactive surface area than Cu2O/ILGS-50, Cu2O/ILGS-12.5 and Cu2O/ILGS-25. It is worth noting that Cu2O nanocubes in Cu2O/ILGS-100present the smallest size of 72 nm and the highest loading amountof 77.6wt%, indicating this interface-induced method has a greatpotential in scaling up synthesis of small size of Cu2O with highconcentrations of Cu2þ. This work not only paves the way for scalepreparation of well-defined Cu2O nanocubes as efficient catalyst forgreen synthesis of ethylene from CO2RR, but also enlightens a facileway to control the morphology and size of metal oxides for variouselectrochemical applications.

Acknowledgements

This work is financially supported by the National Natural Sci-ence Foundation of China (Nos. 21808242, 51572296); the NaturalScience Foundation of Shandong Province (ZR2018BB070), theFinancial Support from Taishan Scholar Project (No. ts201712020).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.electacta.2019.03.146.

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